Troponin complex (troponin T, troponin I, troponin C) plays an important role in the Ca^2+ - dependent regulation of muscle contraction. To study the proximity relationships of the troponin T-I-C subunits, a series mutation of Tin and K fragment of TnT have been made, expressed in E.coli and purified. The single-Tap mutant K fragment of TnT (TnT/K W265F) will work as the donor for distance measurements by fluorescence energy transfer. The distances between Tap-212 of TnT/K to Tin-Cys-96 (N96C), Tin-Cys-117 (S117C), TnC-Met-25 and TnC-Cys-98 will be measured.